Support `preview` command

[ihostage]

This command should do the next steps:

• Clean dist and build folder (maybe clean-webpack-plugin can help us)
• Build UI bundle (currently it is generated by npx webpack command)
• Generate example site (currently it is generated by npx antora antora-playbook.yml command)
• Start local HTTP server and publish build/site directory
• [Optionally] If it's possible to think about hot-reload example site by change of CSS/JS/hbs sources